1. Field of the Invention
This invention relates to a process for reducing the aldehyde concentration in a target stream. The invention, in particular, relates to a process for reducing the aldehyde concentration in a target stream of a carbonylation process. The invention also relates to a process for reducing the aldehyde concentration in a target stream comprising a carbonylatable reactant. More particularly, this invention relates to a process for reducing the aldehyde concentration in a feed stream to a carbonylation reactor of a carbonylation process.
2. Technical Background
Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of a carbonylatable reactant(s), in particular methanol, with carbon monoxide as taught in U.S. Pat. No. 3,769,329, issued to Paulik et al. on Oct. 30, 1973. The carbonylation catalyst typically contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms. Likewise, the nature of the halide promoter is not generally critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
An improvement in the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in commonly assigned U.S. Pat. No. 5,001,259, issued Mar. 19, 1991; U.S. Pat. No. 5,026,908, issued Jun. 25, 1991; and U.S. Pat. No. 5,144,068, issued Sep. 1, 1992; and European Patent No. EP 0 161 874 B2, published Jul. 1, 1992. As disclosed therein, acetic acid is produced from methanol in a reaction medium containing methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically effective concentration. These patents disclose that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e., 4 weight percent or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14-15 wt % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium and at least a finite concentration of water, a specified concentration of iodide ions over and above the iodide ion that is present as hydrogen iodide. This iodide ion is a simple salt, with lithium iodide being preferred. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid, especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt %, so low that it can broadly be defined simply as “a finite concentration” of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e., resistance to catalyst precipitation, especially during the product recovery steps of the process. In these steps, distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide, which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Pat. Nos. 5,001,259, 5,026,908 and 5,144,068 are herein incorporated by reference.
It has been found that although a low water carbonylation process for producing acetic acid reduces such by-products as carbon dioxide, hydrogen, and propionic acid, the amount of other impurities, present generally in trace amounts, can be increased by a low water carbonylation process, and the quality of acetic acid sometimes suffers when attempts are made to increase the production rate by improving catalysts, or modifying reaction conditions.
These trace impurities affect quality of acetic acid, especially when they are recirculated through the reaction process, which, among other things, can result in the build up over time of these impurities. Unsaturated aldehydes, notably crotonaldehyde and ethyl crotonaldehyde, which are derived from aldol reactions of acetaldehyde, are impurities that decrease the permanganate time of the acetic acid, a quality test commonly used in the acetic acid industry. As used herein, the phrase “aldehyde” is intended to mean compounds that contain aldehyde functional groups, which compounds may or may not possess unsaturation. See Catalysis of Organic Reaction, 75, 369-380 (1998), for further discussion on impurities in a carbonylation process. Such aldehyde species may be found in any number of streams of the carbonylation process. As disclosed, such aldehyde species can be generated within the process. As such, aldehyde species may be found within any number of internal process streams. Such aldehyde species are also often found in feed materials commonly used for the carbonylation process. Common industrial sources of carbonylatable reactants may contain aldehyde species, which may be present in undesirable concentrations. As such, aldehyde species may be found in a feed stream provided to the carbonylation reactor. Such streams, e.g., process and feed streams, of the carbonylation process will typically contain at least one carbonylatable reactant.
The present invention is directed to reducing aldehyde concentration, which may be present as compounds such as acetaldehyde, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde and the like, particularly in a target stream comprising a carbonylatable reactant or a target stream of a carbonylation process.
The aldehyde species described above, such as acetaldehyde, are sources for any number of other undesirable byproducts that may be formed in a carbonylation process. Acetaldehyde, in particular, is a source of propionic acid. By reducing the aldehyde concentration, the present invention may also lead to reduction or removal of such undesirable byproducts. Accordingly, it is a primary objective to reduce aldehyde species, notably acetaldehyde.
Conventional techniques to remove such aldehyde impurities include treating the acetic acid product streams with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like. Such treatments may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the final product. It is also known to remove aldehyde impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine, which reacts with the aldehyde species to form oximes, followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the final product adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.
It has thus become important to identify more economically viable methods of reducing the concentration of aldehyde species within the carbonylation process, including process streams and feed streams containing a carbonylatable reactant, without contaminating the final product or adding unnecessary costs. Accordingly, there remains a need for alternative processes to improve the efficiency of aldehyde reduction. The present invention provides one such alternative solution.